LTC1726I データシートの表示（PDF） - Linear Technology

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LTC1726I

Triple Supply Monitor and µP Supervisor with Adjustable Reset and Watchdog Timer

Linear Technology 

LTC1726

Applications Information

Supply Monitoring

The LTC1726 is a low power, high accuracy triple supply

monitor and watchdog timer. The watchdog and reset

periods are both adjustable using external capacitors.

All three VCC inputs must be above predetermined thresh-

olds for reset not to be asserted. The LTC1726 will assert

reset during power-up, power-down and brownout condi-

tions on any one or all of the VCC inputs.

Upon power-up, either the VCC5/VCC25 or VCC3 pin can

power the drive circuits for the RST pin. This ensures that

RST will be low when either VCC5/VCC25 or VCC3 reaches

1V. As long as any one of the VCC inputs is below its pre-

determined threshold, RST will stay a logic low. Once all of

the VCC inputs rise above their thresholds, the adjustable

reset timer is started and RST is released after the reset

time-out period.

On power-down, once any of the VCC inputs drops below

its threshold, RST is held at a logic low. A logic low of

0.3V is guaranteed until both VCC3 and VCC5/VCC25 drop

below 1V.

are greater than approximately 4.15V. In this manner, the

part can function as a 5V monitor with the 3.3V monitor

disabled.

When monitoring either 3.3V or 5V with VCC3 strapped to

VCC5, (see Figure 1) the part determines which is the ap-

propriate range. The part handles this situation as shown

in Figure 2. Above 1V and below VRT3, RST is held low.

From VRT3 to approximately 4.15V, the part assumes

3.3V supply monitoring and RST is deasserted. Above

approximately 4.15V, the part operates as a 5V monitor.

In most systems, the 5V supply will pass through the 3.1V

to 4.15V region in <200ms during power-up, and the RST

output will behave as desired. Table 1 summarizes the state

of RST at various operating voltages with VCC3 = VCC5.

Table 1. Override Truth Table (VCC3 = VCC5)

INPUTS (VCC3 = VCC5 = VCC)

RST

0V ≤ VCC ≤ 1V

—

1V ≤ VCC ≤ VRT3

0

VRT3 ≤ VCC ≤ 4.15V

1

4.15V ≤ VCC ≤ VRT5

0

VRT5 ≤ VCC

1

3V or 5V/2.5V Power Detect

Since the LTC1726 is a multisupply monitor, it will be

required to assert reset as soon as there is power on any

one of the monitor inputs. Therefore, the part derives its

power from either the VCC3 or VCC5/VCC25 input, whichever

pin has the greatest potential. This ensures the part pulls

the RST pin low as soon as either input pin is ≥1V. The

adjustable input is excluded from being a potential supply

pin because of its 1V nominal operating range.

Override Functions (5V Versions Only)

The VCCA pin, if unused, can be tied to either VCC3 or VCC5.

This is an obvious solution since the trip points for VCC3

and VCC5 will always be greater than the trip point for VCCA.

The VCC5 input trip point is disabled if its voltage is equal

to the voltage on VCC3 ±25mV and the voltage on VCC5 is

less than 4.15V. In this manner the LTC1726-5 behaves

as a 3.3V monitor and the 5V reset function is disabled.

The VCC5 trip point is re-enabled when the voltage on VCC5

is equal to the voltage on VCC3 ±25mV and the two inputs

VCC

3.3V OR 5V

LTC1726-5

1

VCC3

2 VCC5

3

VCCA

RST 6

4 GND

R1

10k

TO SYSTEM

RESET

1726 F01

PINS 5, 7 AND 8 NOT SHOWN FOR CLARITY

Figure 1. Single Supply Monitor with Others Disabled

5

VCC3 = VCC5 = VCCA = 0V TO 5V

10k PULL-UP FROM RST TO VCC3

4 TA = 25°C

3

2

1

0

0

1

2

3

4

5

SUPPLY VOLTAGE (V)

1726 F02

Figure 2. RST Voltage vs Supply Voltage

1726fd

8

For more information www.linear.com/LTC1726

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